This invention generally relates to polymeric dielectric materials, methods of forming circuit subassemblies and circuits that include the polymeric dielectric materials, and the subassemblies and circuits formed thereby.
Polymeric dielectric materials are in wide use in the manufacture of electronic circuits. The polymeric dielectric materials are often supplied to the circuit or device manufacturer as a layer in the form of a circuit subassembly. Such subassemblies are used in many semiconductor packaging applications, such as integrated circuit (IC) substrates, radio frequency (RF) systems, and high-speed digital systems. For example, the circuitry of an IC chip (e.g., a microprocessor, a random access memory, a microcontroller, an application specific integrated circuit, and the like) is typically connected to another element of circuitry through interconnect structures, such as an interposer, substrate, and/or board. To make electronic equipment smaller, faster, lighter, and less expensive, high-density interconnect structures are fabricated to accommodate a large number of conductor paths per unit area. The high-density interconnect structure not only miniaturizes the footprint of the IC package, but also can improve signal integrity, such as noise reduction and low attenuation. One way of producing high-density interconnect structures is using a sequential buildup (SBU) method to produce a circuit subassembly, in particular an SBU circuit subassembly.
Conventional SBU circuit subassemblies have two distinct elements: a core and buildup layers. The core can comprise a layer of a dielectric substrate (e.g., a glass-reinforced epoxy resin as used in printed circuit boards (PCBs)), a conductive metal layer (e.g., copper or aluminum), a ceramic layer, a core adhesive layer, or a PCB comprising a dielectric layer and at least one, specifically two, conductive circuit layers disposed on opposite sides of the dielectric layer or a multilayer PCB having more than 1 dielectric layer and more than two conductive layers. The SBU process typically begins with the core, which serves as the carrier for fabricating the buildup layers and provides mechanical support. The buildup layers, consisting of dielectric layers and wiring layers, are sequentially stacked up alternately on one or both surfaces of the core substrate. The wiring layers consist of a plurality of circuitry patterns that provide various wiring functions. Interlayer connection is provided by laser formed or photo defined conductive vias. In order to interconnect the buildup layers on one side of the core to those on the other side, through-holes in the core substrate are mechanically and/or laser drilled or punched and the holes are plated or conductor filled using standard PCB techniques.
“Circuit subassemblies” as used herein further includes other types of subassemblies, for example bond plies, resin coated conductive layers, unclad dielectric substrate layers, cover films, and circuit laminates. A circuit laminate has a conductive layer, e.g., copper, fixedly attached to the cured polymeric dielectric layer. Double clad circuit laminates have two conductive layers, one on each side of the polymeric dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multilayer circuits are formed by laminating one or more circuits together using bond plies, by building up additional layers with resin coated conductive layers that are subsequently etched, or by building up additional layers by adding unclad dielectric layers followed by additive metallization. On lamination, the uncured or B-staged (partially cured) bond plies, resin coated conductive layers, and buildup layers are cured. After forming the multilayer circuit, known hole-forming and plating technologies can be used to produce useful electrical pathways between conductive layers.
Conventional vertically integrated interconnect circuit subassemblies are composed of distinct circuit subassemblies known as subcomposites. There are typically two types of subcomposites: joining core subcomposites and signal core subcomposites. Both types of subcomposites comprise a conductive power layer disposed between two dielectric layers. A plurality of through-holes can be formed in the subcomposites. For the signal core subcomposites, wiring layers are then disposed on the dielectric layers using a semi-additive process. In the joining core subcomposites, the through-holes are then plated with conducting metal and/or filled with an electrically conductive paste. The subcomposites are aligned on top of one another and adhered or laminated together to form the subassembly. The plated metal and/or electrically conductive paste then forms conductive joints between the joining core subcomposites and the signal core subcomposites, thereby providing multiple electrical pathways through the vertically integrated interconnect subassembly. The subassembly can provide for a higher wiring density than capable in SBU subassemblies by the nature of the z-axis electrical interconnections within the subassembly.
Dielectric materials suitable for use in the above-described circuit subassemblies and circuit materials must meet a variety of stringent requirements. In particular, the demand for smaller and less costly electronics operating in multi-band frequency continues to grow. In some cases, IC packaging has become a bar to achieving further reductions in semiconductor size and increases in frequency. Higher frequencies require dielectric materials with very low loss (Df, also known as loss tangent and dissipation factor). Low loss materials contribute very little to the attenuation of the electrical signal during its transmission properties, which can in turn reduce the IC power requirements and peak junction temperatures. However, conventional dielectric materials can have relatively high loss at the higher (e.g., gigahertz) frequencies. For example, a commonly used buildup dielectric film, type GX13 from Ajinomoto, has a loss of 0.019 at 5.8 GHz (Ajinomoto data sheet on ABF materials dated June 2007), which level can be problematic for many high frequency/high speed applications. In addition, absorbed water in the buildup dielectric can have adverse effects on electrical properties, especially in increasing loss, and thereby creating electrical reliability problems. Therefore, a dielectric material with very low water absorption is desired for present and future buildup applications. The aforementioned GX13 buildup dielectric film has a water absorption of greater than 1% which is unacceptable for many high frequency/high speed applications. Such high water absorption also can lead to thermal reliability problems. Further, the capacitance density of conventional dielectric materials can change as a function of frequency and temperature, which can affect the performance of the package.
Also, the dielectric layers function in part to absorb any thermal or mechanical stresses that occur as a result of coefficient of thermal expansion (CTE) mismatches between the package and the core, or between layers in a core. Ideal dielectric materials for higher performance applications, therefore, generally have a low CTE, combined with low modulus and high elongation, to provide a subassembly that is less likely to be affected by the warping and handling problems that can be associated with CTE mismatches. Other desirable properties for high performance buildup layers include good high temperature stability; good thermal conductivity (heat dissipation), low z-axis CTE, and controlled melt flow. The last is critical in buildup dielectric layers' processing and use.
Dielectric materials in present use, such as the afore-mentioned GX13, have many of the desired properties sought for buildup applications, but do not meet all the desired needs for increasing high frequency/high speed applications. These materials suffer especially from having dielectric loss and water absorption considerably higher than desired and high temperature thermal performance less than desired.
There accordingly remains a need in the art for dielectric materials for use in the manufacture of circuit subassemblies and circuits with a combination of low loss at high frequencies, low water absorption, good high temperature stability, and good mechanical properties. It would be a further advantage if the materials had excellent flame retardance in the absence of halogenated flame retardants, and high thermal conductivity.